Proponents of very-low-carbohydrate diets such as ketogenic diets make the claim that there is a “metabolic advantage” in restricting carbohydrates for enhancing body fat loss. A quick internet search will find statements such as….
The ketogenic diet “involves drastically reducing carbohydrate intake and replacing it with fat. This reduction in carbs puts your body into a metabolic state called ketosis. When this happens, your body becomes incredibly efficient at burning fat for energy.” (1)
“A keto or ketogenic diet is a very low-carb diet, which can help you burn fat more effectively.” (2)
“The ketogenic diet brings on a state of constant fat-burning.” (3)
Behind these claims is a new theory, put forward by author Gary Taubes and physician Dr. David Ludwig, among others. It is based upon the premise that carbohydrate restriction is not only helpful for losing weight but actually a requirement for significant body fat loss. This theory is called the carbohydrate-insulin model of obesity (CIM) (4,5,6). Its advocates claim that the CIM can explain the weight increase observed over the last forty years as the advent of a flood of highly processed foods full of added sugar and refined grains that has caused raised insulin levels, forcing fat cells to store fat.
The traditional concept of obesity is based on imbalance of calories. If calorie intake is higher than calorie utilization, weight is gained. And conversely, if calorie burning is greater than calorie ingestion, weight is lost. The new carbohydrate-insulin model of obesity seeks to turn the traditional view of “calories in, calories out” on its head. The CIM theory proposes that insulin is the fundamental controller of fat in the body, dictating whether fat is stored or burned for energy. If the CIM is true, high-carbohydrate intake increases insulin release, raising insulin levels in the blood which encourages fat storage in adipose tissue and suppresses the release of already existing fat from storage. The brain responds by increasing hunger and decreasing the metabolic rate so that the amount of calories burned is reduced and more fat is stored. And so the number of calories eaten or burned is no longer important because, if the CIM is correct, a person can consume fewer calories but still gain weight. Chronic overeating then would not be the CAUSE of obesity but the RESULT of the process of gaining weight. The carbohydrate-insulin model of obesity postulates that eating a low-carbohydrate diet bestows the “metabolic advantage” of increased oxidation of fat and larger body fat loss (4,5).
This is an interesting idea. If it is true then a very-low carbohydrate diet should be recommended to anyone looking to lose weight. However, the scientific basis of the carbohydrate-insulin model of obesity is questionable at best. And, because the CIM provides clear predictions that can be easily observed, it has been tested. For example, the CIM predicts that eating a low-carbohydrate diet will reduce insulin secretion, increase the release of fat from storage and elevate the oxidation of fat. However, studies have not seen these results materialize. Instead, though insulin release is indeed reduced by eating a low-carbohydrate diet and oxidation of fat does increase slightly, the higher predicted loss of body fat does not occur. In fact, the result of investigations into the CIM is surprisingly contrary to this expectation.
SIGNIFICANTLY MORE BODY FAT IS LOST THROUGH EATING A LOW-FAT DIET THAN THROUGH EATING A LOW-CARBOHYDRATE DIET (7).
The Kevin Hall studies
Two fairly recent, meticulously controlled metabolic studies clearly illustrate these results. These investigations were headed by researcher Kevin Hall PhD., Senior Investigator at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health (NIH), where his research interests are macronutrient metabolism, body weight and composition and energy balance.
The first study, conducted in 2015, looked at the amount of fat lost from eating diets restricting carbohydrate compared with that of diets restricting fat (8). Nineteen obese men and women volunteered to spend two periods of two weeks each in a metabolic ward. The advantage of a metabolic ward study is that all food intake and physical activity are monitored and controlled. In other words, there is no chance of “cheating” on the diet or over-exercising to increase weight loss. During the first five days of the first two-week period subjects ate a typical baseline diet (35% fat, 15% protein and 50% carbohydrate (with about 20% of calories coming from sugar)) and with dietary calories matching energy output. On the succeeding days of the study calories were cut by 30% through restricting either carbohydrates or fat. The restricted-carbohydrate diet consisted of 21% protein, 50% fat and 29% carbohydrate (with 8% sugar content). The restricted-fat diet consisted of 21% protein, 8% fat and 71% carbohydrate (with 35% sugar content). These dietary changes resulted in an average reduction of 800 calories a day for both restricted diets compared to the baseline diet. After two weeks the subjects had a break of 2 to 4 weeks and then they returned to the metabolic ward for the second two-week trial period at which time they ate the alternate diet.
Results were measured using the most sensitive method for detecting a change in body fat. Subjects spent two 24-hour periods a week in a metabolic chamber where the difference between fat intake and net fat oxidation (fat oxidation* minus fat production through de novo lipogenesis**) were measured. A metabolic chamber, also known as a whole-room calorimeter, is an airtight room generally furnished with a bed, sink, toilet, television, computer and an exercise bike. Food is passed through a small, air-locked opening in the wall; leftovers, if any, leave in a similar way. Detailed information is kept concerning the food travelling in and out of the room. Fresh air is circulated into the room and analyzed as it flows out, measuring the content of gases such as carbon dioxide and oxygen. Urine is also collected and analyzed. Through these measurements a detailed look at metabolism appears, including number of calories burned and whether those calories come from fat, protein or carbohydrates. There are only about thirty metabolic chambers in the world (9).
*Fat oxidation is a term used for the “burning” of fat to produce energy.
**De novo lipogenesis is the body’s natural process of producing fats (as fatty acids) from excess carbohydrates. Carbohydrates are the preferred fuel for the human body and they are always burned for energy over fats when they are available. Fats are only burned once the carbohydrates are depleted; unneeded fats are stored in body fat, an easy and efficient process. If there are carbohydrates left over after energy needs are satisfied they are then turned into fat for storage through de novo lipogenesis. De novo lipogenesis is an inefficient process that can waste up to 30% of the calories being stored (10).
Those on the restricted-carbohydrate diet experienced significantly lower numbers of calories burned compared to the baseline diet while those on the restricted-fat diet saw no change in number of calories burned compared to the baseline diet. Over the study period, participants in the restricted-fat diet group lost 463 grams of fat on average compared to 245 grams of fat lost in the restricted-carbohydrate diet group. Mathematical models projecting into the future predicted that the restricted-fat diet group would lose over six pounds more than the restricted-carbohydrate diet group if the experiment was continued for six months (8).
In 2016 Kevin Hall and his team published a second study (11). This time seventeen overweight and obese men lived in the metabolic ward for two periods of one month each. During the first month participants ate a higher-carbohydrate high-sugar baseline diet (BD) consisting of 50% of calories from carbohydrate (including 25% of calories from sugar), 15% from protein and 35% from fat); the second month they switched to an equivalent-calorie ketogenic very-low-carbohydrate low-sugar diet (KD) consisting of 5% of calories from carbohydrate (including 2% of calories from sugar), 15% from protein and 80% from fat. Both the BD and the KD menu contained minimal quantities of processed food and the BD did not include large quantities of added sugars despite the large differences in macronutrient and sugar content. Each subject spent two 24-hour days of each week in the metabolic chamber for measurement of the type and number of calories burned while awake and during sleep.
The results this time around? (11)
Subjects lost weight during the BD diet with the majority of the loss coming from body fat. Introduction of the KD diet initially increased the rate of weight loss, an expected effect because low-carbohydrate diets cause immediate rapid loss of water. However, the loss of fat during this period was low and the rate of weight loss quickly slowed. Body fat loss in particular decreased by more than half when the diet of the subjects was changed over to the KD. Researchers state that “Body fat slowed during the KD and coincided with increased protein utilization and loss of fat-free mass.”(11). In other words, the weight loss that occurred during consumption of the KD diet was the result of the burning of proteins from the body. Subjects lost more lean mass than fat mass.
The carbohydrate-insulin model predicts a greater rate of body fat loss when eating a low-carbohydrate KD diet but the data from this study do not support this prediction. The researchers reported that despite rapid, substantial and persistent reductions in daily insulin secretion while eating the KD, there was a slowing of body fat loss. They concluded that regulation of tissue fat storage is not a simple one governed by insulin alone, but a culmination of the effects of multiple factors.
Critics of the results of these studies protested that the studies did not last long enough for subjects to become “fat adapted”, or, in other words, to convert from using glucose for energy to fats. However, the balance of research on fasting and starvation shows that adaptations to carbohydrate restriction actually occur within a week of the start of a prolonged fast (7).
What what can we learn from these studies?
Low-fat diets are a more efficient way to lose body fat than low-carbohydrate diets.
There is no “metabolic advantage” to be gained from low-carbohydrate diets.
The carbohydrate-insulin model of obesity (CIM) does not hold true.
The recent rise in obesity may simply be due to the consumption of extra calories.
4 Taubes, G. Why we get fat and what to do about it. Alfred A. Knopf, New York (2011).
5 Ludwig, D.S., Friedman, M.I. Increasing Adiposity: Consequence or cause of overeating? JAMA 2014; 311(21): 2167-2168.
6 Ludwig, D.S. Always hungry? Conquer cravings, retrain your fat cells and lose weight permanently. Grand Central Life & Style: New York, 2016.
7 Hall, K.D. A review of the carbohydrate-insulin model of obesity. Eur J Clin Nutr. 2017;71: 323-326.
8 Hall, K., Bemis, T., Brychta, R., et al. Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity. Cell Metab. 2015; 22(3): 427-436.
10 Hellerstein, M.K. De novo lipogenesis in humans: metabolic and regulatory aspects. Eur J Clin Nutr. 1999 Apr;53 Suppl 1:S53-65.
11 Hall, K.D., Chen, K.Y., Guo, J., et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am J Clin Nutr. 2016; 104: 324-333.
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18 Holt, S.H.A, Brand Miller, J.C., Petocz, P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr 66, 1264-1276